Can you ever recover these cards?
Power failures are bad news for a system that is not battery backed up. If you're writing to your SD card when it loses power, the file system could be in all sorts of odd states after the system restarts.
There are various ways to deal with this sort of thing:
1) Use a more robust file system or journalling mode on your existing file system.
2) Have a read-only partition that contains the meat and potatoes of the operating system, and a writeable partition that you don't care if it gets nuked (can always be repaired later).
3) Develop a battery backed system that it is impossible to have a catastrophic power loss on.
4) Combination of the above.
Really, it all depends on my first question; are the cards actually toast physically, or just at a file system level?